It's Linux--you got to roll with it.
I would just
A) rip all passwords out of vivaldi (disable it) and cut and paste from a password manager so you never have use that stupid keyring again.
then B) uninstall/reinstall vivaldi
or
C) use another browser purely for login accounts.
or
I wonder if you monkey with anything in /.config/vivaldi
vivaldi settings?
or
type 'disable vivaldi keyring prompt' into DDG and see if there's an easy fix
or
see if it persists if you make solus autostart it
that is the end of my brain